To the present, known are various methods to apply a liquid coating composition (hereinafter occasionally referred simply to as a coating composition) onto a support. For example, proposed as methods to precisely apply a liquid coating composition onto a conveyed long belt-shaped support (hereinafter occasionally referred simply to as a support) are various methods as described in Edward Cohen and Edgar Gutoff, “MODERN COATING AND DRYING TECHNOLOGY”. For example, known are a dip coating method, a blade coating method, an air knife coating method, a wire bar coating method, a gravure coating method, a reverse coating method, an extrusion coating method, a slide bead coating method, and a curtain coating method. Further, in these coating methods, in order to obtain a dried layer thickness which is highly uniform across the width of the support, coating is performed while paying special attention to the coating thickness accuracy and uniformity (after coating and prior to drying) during coating.
Of these coating methods, a coating apparatus, having flow controlling type die, is capable of achieving high coating rate, thin layers, and simultaneous multilayer coating. Due to such characteristics, the above apparatus is widely used as a coating apparatus for light-sensitive photographic materials, ink-jet recording materials, and magnetic recording materials.
As a preferred example, the slide bead coating apparatus and the extrusion coating apparatus, which are proposed in U.S. Pat. No. 2,761,791 by Russell et al. have been widely used. Further, the curtain coating apparatus is also a flow controlling type coating apparatus having a die and is similarly widely employed.
For example, in the case of the above slide bead coating apparatus, a shaped liquid coating composition, called a bead is formed between the tip of the coating apparatus and the conveyed support, and coating is performed via the bead. Further, in the case of a curtain coating apparatus, a liquid coating composition shaped as a curtain falls freely and coating is performed in such a manner that a support is positioned at the falling front. These are markedly advantageous to obtain a highly accurate and uniformly thick dried layer.
However, during coating employing these coating apparatuses having a die, due to their principle, the coating apparatus and the support are continuously connected via the liquid coating composition, such as a bead or a curtain film. In order to form a uniformly thick coating film on a support, it is essential that a liquid coating composition continuously flows at a constant rate and no discontinuity is allowed to occur. Namely, in order to continuously form a coated film and to maintain a highly accurate coating thickness, the liquid coating composition is required in an amount greater than the specified amount. Accordingly, an excessive decrease in the amount of the liquid coating composition ejected from a coating apparatus results in difficulty to achieve the target to obtain uniform thickness.
Due to that, in the case of a small amount of elusion per coating layer, namely in the case in which a layer of a very thin wet thickness (for example, at about 1—about 50 μm), prior to drying after coating a liquid coating composition is formed, it is required to increase the solvent amount of the liquid coating composition to increase its total volume. Specifically, in cases in which the viscosity of liquid coating compositions is low, the coated composition flows on a support resulting in an unstable coating layer, whereby it is required to further increase the amount of the liquid coating composition.
However, in view of production efficiency, an increase in the solvent amount is not preferred due to an increase in load (being a drying load) to dry the coating by evaporating the excessive solvents after coating. Further, in cases in which another constituting layer is present below the aforesaid coating layer, the liquid coating composition of the aforesaid coating layer excessively penetrates and diffuses into the upper constituting layer, occasionally resulting in adverse effects.
Consequently, a coating method is demanded which provides a thin layer which achieves higher accuracy in a coating layer thickness, less drying load, and higher production efficiency.
There are various coating products which require to provide such a uniform thin layer of a high accuracy on a constituting layer. For example, listed are the porous ink-jet recording sheet described below.
Recording sheet used for ink-jet recording include one in which the ink absorptive layer is paper itself such as plain paper, another one in which an ink absorptive layer is coated onto a support such as coated paper, which also works as an absorbent, or still another one in which an ink absorptive layer is applied onto a non-absorptive support such as a resin coated paper or a polyester film.
Of these, recording sheet in which an ink absorptive layer is applied onto a non-absorptive support are preferably employed for output which requires a feel of high quality such as a feel of gloss, a feel of luster, or feel of depth of silver halide photography due to reasons in which the support surface exhibits high smoothness and minimal waviness. Further, employed as glossy type recording sheet are swelling type recording sheet in which water-soluble binders such as polyvinylpyrrolidone and polyvinyl alcohol are applied onto a non-absorptive support as an ink absorptive layer, and so-called porous rerecording sheet in which a minute porous structure is formed in an ink absorptive layer employing pigments, or pigments, and binders and ink is absorbed into the resulting voids.
In a porous recording sheet, an ink absorptive porous layer having the above porous structure is formed mainly by employing hydrophilic binders and microparticles (hereinafter it is also called as microparticles). Known as microparticles are inorganic or organic ones, while inorganic microparticles are commonly employed which are minuter and glossier. By employing hydrophilic binders in a relatively small amount with respect to the above microparticles, minute voids are formed among the microparticles, whereby a porous ink absorptive layer is prepared.
Various characteristics are demanded for the above porous ink absorptive layer, and in order to improve these various characteristics, it is proposed to use each of the additives described below.
Listed are:    1: in order to achieve high color forming efficiency and desired glossiness, stable microparticles which form porosity of at most approximately 0.1 μm,    2: low swelling hydrophilic binders which exhibit high minute particle capturing power and result in no decrease in the ink absorption rate,    3: cross-linking agents to enhance the ink absorption rate and to improve the waterfastness of layers,    4: surface active agents and hydrophilic polymers distributed over the surface to achieve optimal dot diameter,    5: cationic fixing agents and multivalent metal compounds to minimize dye bleeding and to enhance waterfastness,    6: anti-discoloring agents to minimize discoloration of dye images due to ambient light and oxidizing gases    7: optical brightening agents and color tone controlling agents (reddening agents and bluing agents) to improve white backgrounds,    8: matting agents and slipping agents to improve slipping properties of the surface,    9: various types of oil components, latex particles, or water-soluble plasticizers to provide the porous ink absorptive layers with flexibility,    10: various inorganic salts (multivalent metal salts) to minimize dye bleeding and enhance waterfastness and weather resistance, and    11: acids and alkalis which control the pH of the surface of the porous ink absorptive layer.
However, when additives which are employed to achieve the various above targets are added to liquid coating compositions which form the porous ink absorptive layer, in many cases, additives exhibit various types of limitation in view of stability of the production processes.
As one of the methods to overcome the above drawbacks, a porous ink absorptive layer liquid coating composition, in which the above additives are not incorporated, is initially applied as a constituting layer onto a support, and prior to reaching the falling-rate drying, a liquid coating composition incorporating the above additives is applied onto the above constituting layer, namely a so-called overcoat layer is provided (refer, for example, to Patent Documents 1 and 2). It is thought that the above additives incorporated in the liquid coating composition appropriately penetrate into the previously provided constituting layer (for example, a porous ink absorptive layer) and exhibit desired functions without causing the above drawbacks, namely working as a function providing compound. Originally, the overcoat layer is employed so that specific function providing compounds are impregnated into the porous ink absorptive layer. Consequently, the thickness of the overcoat layer may be quite thin, and is rather preferably very thin. Further, ink-jet recording sheet are proposed (refer, for example, to Patent Document 3) in which a water-based coating composition which incorporates hydrophilic binders and microparticles is coated and after the water volume in the resulting coating reaches that which is equal to or less than the void volume of the porous layer after drying, a solution incorporating additives is overcoated via on-line.
However, when two layers, consisting of a constituting layer and an overcoat layer, are provided employing the following two processes, problems occur in which production cost increases markedly. The above constituting layer is initially coated and dried, and the resulting coating is temporarily wound in a roll. Thereafter, the coating is unwound and the above overcoat layer is applied thereon. Further, when time is elapsed after forming the constituting layer, problems tend to occur in which stability of product quality is degraded due to temperature hysterisis as well as time fluctuation, and coating mottle tends to occur during providing the overcoat layer.
As a result, coating the overcoat layer supplies a large amount of solvents (water and organic solvents) onto the surface of the ink absorptive layer, whereby cost increases due to extension of drying time and length of the drying zone, and when drying capacity is limited, the coating rate inevitably decreases. Further, by coating an overcoat layer of excessive thickness, the degree of diffusion and penetration into the ink absorptive layer during the period until drying increases, and it takes time to achieve complete drying. As a result, effects result so that additives are directly incorporated in the porous ink layer liquid coating composition, whereby it is not possible of the overcoat layer to sufficiently exhibit the desired advantages.
In order to overcome the above drawbacks, the following coating method is proposed (refer, for example, to Patent Document 4). While conveying a medium to be coated, by employing a slot nozzle spray device, fitted with a liquid coating composition nozzle which supplies a liquid coating composition and a gas ejecting nozzle which is near the aperture end of the liquid coating composition nozzle over the coating width in the direction crossing with the conveying direction of the medium to be coated, gas is allowed to collide with the liquid coating composition to form droplets in the form of spray, whereby the liquid coating composition is applied onto the medium to be coated.
The inventors of the present invention further conducted detailed investigation of the method described in Patent Document 4. As a result, it was discovered that the above method made it possible to realize a thin uniform thickness layer coating of high accuracy, high rate and low drying load, and exhibited various excellent characteristics of recording sheet, such as coating uniformity, and liquid coating composition stability. However, it was also discovered that depending on specified coating conditions, particle-shaped mottle due to liquid coating composition droplets, longitudinal streaking mottle or spot-shaped mottle due to scattering of coarse droplets tended to occur. In order to overcome these drawbacks, it is necessary to optimize physical properties of the ejected overcoat liquid coating composition, the shape of the employed coating apparatus, and the surface treatments of the specified positions.                (Patent Document 1) Japanese Patent Publication for Public Inspection (hereinafter referred to as JP-A) No. 11-115308 (claims)        (Patent Document 2) JP-A No. 11-192777 (claims)        (Patent Document 3) JP-A No. 2002-331745 (claims)        (Patent Document 4) JP-A No. 2004-906 (claims)        